LiDAR system comprising a single-photon detector

ABSTRACT

A method for developing a map of objects in a region surrounding a location is disclosed. The method includes interrogating the region along a detection axis with a series of optical pulses and detecting reflections of the optical pulses that originate at objects located along the detection axis. A multi-dimensional map of the region is developed by scanning the detection axis about the location in at least one dimension. The reflections are detected via a single-photon detector that is armed using a sub-gating scheme such that the single-photon detector selectively detects photons of reflections that originate only within each of a plurality of zones that collectively define the detection field. In some embodiments, the optical pulses have a wavelength within the range of 1350 nm to 1390 nm, which is a spectral range having a relatively high eye-safety threshold and a relatively low solar background.

CROSS REFERENCE TO RELATED APPLICATIONS

This case is a continuation of co-pending U.S. patent application Ser.No. 14/147,478 filed Jan. 3, 2014, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to laser range finding in general, and,more particularly, to LiDAR.

BACKGROUND OF THE INVENTION

Light Detection And Ranging (LiDAR) systems are attractive for use inmany applications, such as driverless automobiles, farm equipment, andthe like. Laser range finding is used to determine the range from asource to an object by sending a pulse of light in the direction of theobject, detecting a reflection of the pulse, and determining the timerequired for the light to travel to and return from the object (i.e.,its time-of-flight).

A typical prior-art LiDAR system creates a local map around a vehicle byperforming laser range finding in several directions and elevationsaround the vehicle. Prior-art systems accomplish this in different ways,such as using an array of laser sources, rotating a single laser sourceabout an axis through the vehicle, or directing the output signal from asingle source about the vehicle using a rotating mirror or prism, or astationary reflective cone. For example, U.S. Patent Publication No.20110216304 describes a LiDAR system based on a vertically orientedarray of emitter/detector pairs that are rotated about an axis toprovide a 360° horizontal field-of-view (FOV) and vertical FOV ofseveral tens of degrees. This system emits multiple pulses at a highrepetition rate while the emitter/detector assembly is scanned about thevehicle. The resultant distance measurements form the basis for athree-dimensional simulated image of the scene around the vehicle.

The requirements for a LiDAR system used in automotive applications arequite challenging. For instance, the system needs to have a large FOV inboth the horizontal and vertical directions, where the FOV is supportedover a distance that ranges from approximately 5 meters (m) toapproximately 300 m. Further, the system must have high resolution, aswell as an ability to accommodate a changing environment surrounding avehicle that could be travelling at relatively high speed. As a result,the system needs to be able to update the simulated environment aroundthe vehicle at a high rate. In addition, an automotive LiDAR systemneeds to be able to operate at both day and night. As a result, thesystem needs to accommodate a wide range of ambient light conditions.

The need for high-resolution performance would normally dictate the useof high laser power to ensure sufficient return signal from objects asfar away as 300 m. Unfortunately, eye safety considerations limit thelaser power that can be used in a LiDAR system. The safety threshold forthe human eye is a function of wavelength, with longer wavelengthsbeing, in general, safer. For example, the Maximum Permissible Energy(MPE) for a nanosecond-range light pulse is approximately six orders ofmagnitude higher at 1550 nanometers (nm) than at 980 nm. Unfortunately,the solar background is quite high at 1550 nm and can degrademeasurement sensitivity in this wavelength regime. As a result, manyprior-art LiDAR systems operate in the wavelength range fromapproximately 800 nm to approximately 1050 nm at low optical powerlevels, thereby sacrificing system performance.

Some prior-art LiDAR systems do operate 1550 nm; however, FOV isnormally restricted to mitigate the effects of the solar backgroundradiation. Unfortunately, such an operational mode reduces the updaterate for the system. To develop a sufficiently large image of thesurrounding region, therefore, multiple systems having overlappingfields of view are required, which increases overall cost and systemcomplexity.

For these reasons, a low-cost, high-performance LiDAR system suitablefor vehicular applications would represent a significant advance in thestate-of-the-art.

SUMMARY OF THE INVENTION

The present invention enables a LiDAR system without some of the costsand disadvantages of the prior art. Embodiments of the present inventionare particularly well-suited for use in applications such as autonomousvehicles, adaptive automotive cruise control, collision-avoidancesystems, and the like.

An illustrative embodiment of the present invention is a LiDAR systemcomprising a transmitter and a single-photon detector, where transmitterprovides nanosecond-scale optical pulses that are in a wavelength rangefrom approximately 1350 nm to approximately 1390 nm. Operation in thiswavelength regime exploits an MPE that is several orders of magnitudehigher than at the 905 nm wavelength typically used in prior-art LiDARsystems. Also, by operating in this wavelength range, embodiments of thepresent invention can also take advantage of a narrow wavelength band inwhich the solar background is relatively low. In order to enable an FOVthat extends to a distance of 500 m, the single-photon detector is gatedat a frequency of approximately 3.3 MHz, yielding a series of 0.3microsecond detection frames.

To further reduce the impact of solar-background-induced noise, in someembodiments, the single-photon detector is operated in a “range-gated”mode. In such operation, each detection frame includes a plurality ofsub-gate periods, each including a different portion of the detectionframe. The single-photon detector is armed at the beginning of eachsub-gate period and disarmed at the end of each sub-gate period. As aresult, each of the sub-gate periods corresponds to a longitudinal sliceof the detection field. By cycling through a series of detection frames,each including the plurality of sub-gate periods, the present inventionenables high signal-to-noise operation with high resolution over theentire field-of-view. By using short sub-gate periods, the probabilityof detecting a noise photon associated with the solar background in anyindividual sub-gate period is dramatically reduced.

In some embodiments, the results from multiple detection frames areprocessed using statistical analysis techniques to reduce the impact ofthe receipt of noise photons. In some embodiments, the results frommultiple detection frames are collected in a dataset and digitalthresholding is applied to the dataset to mitigate the effects ofbackground noise.

An embodiment of the present invention comprises a method for developinga map of objects relative to a first location, the method comprising:transmitting a first optical pulse at a first time, the first opticalpulse having a wavelength within the range of approximately 1350 nm toapproximately 1390 nm; detecting a first reflection of the first opticalpulse from a first object within the detection field, wherein the firstreflection is detected at a second time; and computing a distancebetween the first location and the first object based on the differencebetween the first time and the second time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic drawing of a LiDAR system in accordance withthe illustrative embodiment of the present invention.

FIG. 2 depicts operations of a method for developing a map of objects ina detection field in accordance with the illustrative embodiment of thepresent invention.

FIG. 3 depicts a plot of Maximum Permissible Energy (MPE) versuswavelength for pulses having different pulse widths.

FIG. 4 depicts a plot of solar power density as a function ofwavelength.

FIG. 5 depicts a circuit diagram of a portion of a receiver inaccordance with the illustrative embodiment of the present invention.

FIG. 6 depicts a timing diagram for receiver operation of a LiDAR systemin accordance with the illustrative embodiment of the present invention.

FIG. 7 depicts a schematic drawing of a LiDAR system in accordance witha first alternative embodiment of the present invention.

FIG. 8 depicts operations suitable for developing a map of a detectionfield in accordance with the first alternative embodiment of the presentinvention.

FIG. 9 depicts a sub-method suitable for interrogating a detection fieldalong a detection axis in accordance with the first alternativeembodiment of the present invention.

FIG. 10 depicts a timing diagram for interrogating a detection fieldalong a detection axis in accordance with the first alternativeembodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 depicts a schematic drawing of a LiDAR system in accordance withthe illustrative embodiment of the present invention. System 100includes transmitter 102, receiver 104, and processor 106.

FIG. 2 depicts operations of a method for developing a map of objects ina detection field in accordance with the illustrative embodiment of thepresent invention. Method 200 begins with operation 201, whereintransmitter 102 transmits a train of pulses 110 along detection axis112.

Transmitter 102 is an optical transmitter suitable for emitting a trainof optical pulses having a wavelength within the range of approximately1350 nm to approximately 1390 nm. Typically transmitter 102 generatesthe optical pulses using a laser source, such as a diode laser.

One skilled in the art will recognize that the performance of a LiDARsystem is based on several factors, such as available optical power, thesensitivity of the detector used to detect reflected pulses, and thedesired sensitivity of the system. The present invention employssingle-photon detectors, which greatly reduces the amount of opticalpower required in pulses 110. Significant optical power is stillrequired to enable a suitable signal-to-noise ratio (SNR) for longerdetection distances, however. Unfortunately, the amount of optical powerthat can be used in a LiDAR system is constrained by eye-safety issues.These are particularly stringent at shorter wavelengths, such as at 905nm, a typical operating wavelength of prior-art LiDAR systems.

It is an aspect of the present invention that operation at a wavelengthwithin the 1350-1390 nm-wavelength range enables embodiments of thepresent invention to take advantage of a relatively narrow intensity dipin the solar background spectrum. This wavelength range also exploits aneye-safety threshold that is more than an order of magnitude higher thanat 905 nm.

FIG. 3 depicts a plot of Maximum Permissible Energy (MPE) versuswavelength for pulses having different pulse widths. One skilled in theart will recognize that the MPE denotes an eye-safety threshold for thepower at which a LiDAR system can safely operate.

It is apparent from plot 300 that, for nanosecond pulse widths, MPE isapproximately 1 microJoule/cm² at a wavelength of 900 nm; however, itincreases to approximately 50, 5000, and 10⁶ microJoules/cm² at thewavelengths of 1350, 1450, and 1550 nm, respectively. By operating atlonger wavelengths, therefore, system 100 can provide higher laser powerwithout exceeding the eye safety threshold.

It should be noted that plot 300 also suggests that operation at 1550 nmshould enable even longer ranging distance, as well as improved systemperformance. Unfortunately, the solar background radiation is alsohigher at 1550 nm wavelengths, thereby degrading the signal-to-noiseratio and negating many of the benefits of long-wavelength operation.

FIG. 4 depicts a plot of solar power density as a function ofwavelength.

While plot 400 evinces that the power level of the solar background isquite high at 1550 nm, it also shows that it dips significantly withinthe range of approximately 1340 nm to approximately 1500 nm. Trace 402indicates a sliding 40 nm-wide average of the solar energy. Window 404denotes a 40 nm-wide wavelength window suitable for vehicular LiDARoperation, which is centered at 1370 nm in accordance with theillustrative embodiment of the present invention. It should be notedthat other operational windows are in accordance with the presentinvention. For example, plot 400 also depicts window 406, centered atapproximately 1455 nm, which represents another possible window ofoperation for system 100. Operation in this wavelength regime allowspulse 110 to be at a higher energy level by virtue of a higher MPE atthis higher wavelength.

Transmitter 102 transmits each of pulses 110 such that it has a durationof approximately 5 nanoseconds at a repetition rate of approximately 300kHz. In some embodiments, each of pulses 110 is transmitted with aduration that is less than 5 ns. In some embodiments, each of pulses 110is transmitted with a duration that is greater than 5 ns. One skilled inthe art will recognize that the pulse width of pulse 110 and therepetition rate of transmitter 102 are matters of design and can thatpulse 110 can have any suitable values.

In the illustrative embodiment, the repetition rate of transmitter 102is based on the desired maximum range of detection, Lmax, in detectionfield 114 (i.e., the size of detection field 114). In the illustrativeembodiment, transmitter 102 transmits a train of pulses 110 at arepetition rate of 3.3 microseconds, which is based on thetime-of-flight for a photon travelling to and from an object that is ata maximum detection distance that is approximately 500 meters fromreceiver 104. This time-of-flight also determines the duration of eachof a plurality of detection frames for system 100, as discussed belowand with respect to FIG. 5.

It should be noted that, in the illustrative embodiment, system 100 ismounted on vehicle 108 such that transmitter 102 and receiver 104 aresubstantially co-located. In some embodiments, transmitter 102 andreceiver 104 are not co-located and are separated by some separationdistance and the repetition rate of transmitter 102 is based on thetotal time for a photon to travel from transmitter 102 to an objectlocated at Lmax and be reflected back to receiver 104.

At operation 202, processor 106 enables receiver 104 to detectreflection 116.

FIG. 5 depicts a circuit diagram of a portion of a receiver inaccordance with the illustrative embodiment of the present invention.Receiver 104 comprises single-photon avalanche diode (SPAD) 502 and load504. In some embodiments, receiver 104 includes a negative-feedbackavalanche diode (NFAD), a SPAD operatively coupled with a switchingelement (e.g., a transistor) operative for discharging the SPAD after anavalanche event, a photodiode operating in linear mode, or anothersuitable photodiode. SPADs and NFADs suitable for use in the presentinvention are described in detail in U.S. Pat. Nos. 7,378,689 and7,719,029, each of which is incorporated herein by reference.

FIG. 6 depicts a timing diagram for receiver operation of a LiDAR systemin accordance with the illustrative embodiment of the present invention.Each detection frame depicted in timing diagram 600 (i.e., detectionframe 602-1) begins at time t0, when transmitter 102 transmits pulse110.

At time t1, processor 106 arms SPAD 502 by applying a voltage bias, V2,at terminal 506, where V2 exceeds breakdown voltage Vbr of SPAD 502. Forthe purposes of this Specification, including the appended claims,“arming” a SPAD is defined as putting the diode into Geiger mode bybiasing it with a bias voltage that exceeds the breakdown voltage of theSPAD. One skilled in the art will recognize that when armed, receipt ofa single photon will give rise to an avalanche event that results in amacroscopically detectable current through the SPAD. In similar fashion,“disarming” a SPAD is defined taking it out of Geiger mode by reducingits bias voltage below the breakdown voltage of the SPAD. It should benoted that time t1 is delayed slightly from time t0 to account for thetransmission duration of pulse 110, during which detection of reflection116 is disabled. In some embodiments, SPAD 502 is armed simultaneouslywith, or during, the transmission of pulse 110.

Prior to absorbing a photon, the entirety of Vbias is developed acrossSPAD 504 since the magnitude of avalanche current, i, is zero and thereis no voltage drop across load 504. As a result, the magnitude of Vout(provided to processor 106 at terminal 508) is equal to zero. Althoughthe illustrative embodiment comprises a load that is a resistive load,in some embodiments of the present invention, load 504 comprises acircuit element in addition to or other than a resistor, such as acapacitor, inductor, or reactive element (e.g., a transistor, etc.).

At operation 203, receiver 104 detects reflection 116, which originatesfrom object 118. Reflection 116 is detected at receiver 104 at time tr.

Upon absorbing a photon, avalanche current, i, builds through SPAD 502and load 504. As a result, a voltage drop equal to i·R will appearacross load 504, where R is the resistance of load 504. As a result,processor 106 detects a change in the magnitude of voltage Vout, at timetr.

At operation 204, processor 106 computes the distance between vehicle108 and object 118 based on the delay between times t0 and tr.

It should be noted that the use of an NFAD in receiver 104 enablesdetection of multiple reflections within a single detection frame, sincean NFAD can be rapidly quenched and re-armed at rates higher than thetypical repetition rate of transmitter 102, as discussed in detail inU.S. Pat. No. 7,719,029.

At operation 205, processor 106 disarms SPAD 502 at time t2 by reducingits bias voltage below Vbias. It should be noted that time t2 indetection frame 602-1 is also time t0 of detection frame 602-2.

In some embodiments, operations 201 through 205 are performed for eachof detection frames 602-i, where i=1 through N and N is a desired numberof detection frames over which it is desired to interrogate detectionfield 114 along detection axis 112. Repeated interrogation of the samedetection axis enables the use of statistical methods to increase systemtolerance for false counts at SPAD 502. In some embodiments, digitalthresholding is used to increase system tolerance for false counts, asdiscussed below and with respect to FIGS. 7-10.

At operation 206, system 100 scans detection axis 112 horizontally suchthat system 100 interrogates a different portion of detection field 114.In some embodiments, system 100 scans detection axis 112 vertically.Typically, detection axis 112 is scanned via a rotating element, such asa mirror, prism, and the like. In some embodiments, transmitter 102 andreceiver 104 are mounted on a scanning element, such as a turntable,gimbal element, etc., which enables detection axis 112 to be scanned.

In some embodiments, system 100 includes a one- or two-dimensional arrayof transmitters and/or receivers that are arranged to simultaneouslyinterrogate detection field 114 along a plurality of detection axesarranged horizontally and/or vertically.

At operation 207, processor 106 develops a map of detection field 114.

It should be noted that even when system 100 operates at a wavelengthwithin the range of 1350-1390 nm, the power level of the solarbackground is still quite high and can lead to false counts at receiver104, as shown in Table 1 below.

TABLE 1 Maximum Solar-Power False Photon Wavelength Density on Countsper Nanosecond Range Ground Level High FOV Res. Low FOV Res. (nm)(W/m²/nm) 0.075 deg 0.15 deg  905 ± 20 0.7 12 (2500 pW) 46 (100000 pW)1370 ± 20 10⁻⁴ 0.0025 (0.4 pW) 0.01 (1.6 pW) 1420 ± 20 2 × 10⁻² 0.5 (72pW) 2 (290 pW)

Although the false-count rate at 1370 nm is much lower than at otherwavelengths, even at 1370 nm, 6-30 false counts will occur in adetection frame of approximately 3 microseconds.

It is another aspect of the present invention that the negative impactof solar background on system performance is mitigated by apportioningeach detection frame into a plurality of sub-gate periods. In otherwords, by dividing each detection frame into smaller time intervals inwhich SPAD 502 is armed, the probability of a false count during eachsub-gate period can be greatly reduced, as shown in Table 2 below.

TABLE 2 False Photon Counts per Sub-gate Sub-gate Periods Per High FOVRes. Low FOV Res. 1 microsecond-long Detection Frame 0.075 deg 0.15 deg20 (50 ns/sub-gate) 0.1 0.5 50 (20 ns/sub-gate) 0.05 0.2

FIG. 7 depicts a schematic drawing of a LiDAR system in accordance witha first alternative embodiment of the present invention. System 700 isanalogous to system 100; however, in operation, processor 702 isoperative for logically separating detection field 114 into detectionzones 704-1 through 704-3 (referred to, collectively, as zones 704)—eachof which corresponds to a different sub-gate period within eachdetection frame. In the first alternative embodiment, detection field114 is separated into three zones; however, it will be clear to oneskilled in the art, after reading this Specification, how to specify,make, and use embodiments of the present invention wherein detectionfield 114 is separated into any practical number of zones.

FIG. 8 depicts operations suitable for developing a map of a detectionfield in accordance with the first alternative embodiment of the presentinvention. Method 800 begins with operation 801, wherein, for each ofi=1 through N, system 700 interrogates detection field 114 alongdetection axis 112 during detection frame 1002-i. N is the number oftimes interrogation of detection axis 112 is repeated before rotatingdetection axis 112 to enable system 100 to interrogate a differentportion of detection field 114.

The value of N is based on a number of factors. For example, in anyLiDAR system, it is important to be able to distinguish detection of asignal photon (i.e., a photon arising from reflection from an object indetection field 114) from a detected “noise photon” due to the solarbackground, a dark count, and the like. For proper system operation, aProbability of Detection (PD) of a signal photon is preferably at least99%, while the false alarm rate (FAR) due to noise photons is preferably<1%. It is yet another aspect of the present invention that, byinterrogating detection axis 112 repeatedly to collect data frommultiple detection frames, statistical methods and/or digitalthresholding can be employed to increase PD and decrease FAR, asdiscussed below.

FIG. 9 depicts a sub-method suitable for interrogating a detection fieldalong a detection axis in accordance with the first alternativeembodiment of the present invention. Operation 801 begins withsub-operation 901, wherein transmitter 102 transmits pulse 110 alongdetection axis 112.

FIG. 10 depicts a timing diagram for interrogating a detection fieldalong a detection axis in accordance with the first alternativeembodiment of the present invention. In similar fashion to timingdiagram 600 described above and with respect to FIG. 6, each measurementframe 1002-i begins at time t0-i, when transmitter 102 transmits pulse110-i.

For each of j=1 through M, where M is the number of sub-gate periodswithin each detection frame, processor 106 enables receiver 104 todetect a reflection that originates only within zone 704-j.

Detection of a reflection originating within each zone 704-j is enabledat sub-operation 902, wherein SPAD 502 is armed at time ta-i-j. Timeta-i-j corresponds to the time-of-flight for a photon between vehicle108 and point 706-j (i.e., from transmitter 102 to point 706-j and backto receiver 104), which is the point in zone 704-j nearest vehicle 108on detection axis 112.

At sub-operation 903, reflection 712-j is detected at SPAD 502 atdetection time tr-i-j. It should be noted that a reflection is notalways detected from each zone 704. For example, in the example depictedin FIGS. 7-10, objects are located only in zones 704-2 and 704-3. As aresult, no reflection is originated in zone 704-1 and Vout remains atzero volts throughout sub-gate period 1004-i-1.

At sub-operation 904, detection time tr-i-j is saved in memory atprocessor 702.

As discussed above and with respect to system 100, the use of an NFAD inreceiver 104 enables detection of multiple reflections within a singledetection frame, or in the case of system 700, within a single sub-gateperiod.

At sub-operation 905, SPAD 502 is disarmed at time td-i-j. Time td-i-jcorresponds to the time-of-flight for a photon between vehicle 108 andpoint 708-j, which is the point in zone 704-j furthest from vehicle 108on detection axis 112.

In some embodiments, SPAD 502 is not armed in each sub-gate periodwithin each detection frame. For example, in some embodiments, a SPAD isarmed only during a subset of the sub-gate periods (e.g., only duringsub-gate period 1004-i-1) for a first plurality of detection frames andarmed during a different subset of the sub-gate periods (e.g., onlyduring sub-gate period 1004-i-2) for a second plurality of detectionframes, and so on.

By dividing each detection frame into a plurality of sub-gate periods,embodiments of the present invention—particularly those embodiments thatoperate at a wavelength within the range of 1350 nm to 1390 nm—areafforded advantages over prior-art LiDAR systems that operate at otherwavelengths because of a significant reduction in the probability ofdetecting a photon due to the solar background. For example, prior-artLiDAR systems operating at a wavelength of 905 nm struggle to overcomethe effects of the solar background since their detectors are blinded bysolar-based photons on a substantially continuous basis. Further,prior-art LiDAR systems that operate at longer wavelengths, such as1550, typically must deal with solar background by restricting FOV ofeach pixel and/or by using very narrow bandwidth filters. For vehicularLiDAR, however, FOV requirements are high. As a result, morerestricted-FOV pixels must be included to cover the same system-levelFOV. In addition, it is challenging to provide cost-effective wavelengthfilters suitable for filtering out solar background, since the filterbandwidth cannot be narrower than the spectrum of the laser used totransmit pulses 110. Embodiments of the present invention avoid the needfor restricted FOV as well as wavelength filters and, therefore, affordbetter system performance and lower cost.

Returning now to method 800, at operation 802, digital thresholding isapplied to dataset 714. Digital thresholding enables an improvement inthe Probability of Detection (PD) of a signal photon while also reducingthe impact of “noise photons,” such as detected photons arising fromevents other than reflection from an object (e.g., a solar backgroundphoton, dark count, and the like). The negative impact of noise photonson system performance is mitigated by setting a threshold value for thenumber of reflections detected from a zone during a plurality ofdetection frames (e.g., detections frame-1; through detection frame-N),and establishing a position for an object in that zone only when thenumber of reflections detected is equal to or greater than the thresholdvalue.

The positive impact of applying digital thresholding to dataset 714 canbe readily seen in Table 3 below.

TABLE 3 Data Collected Over 20 Detection Frames to Enhance SNR Threshold# of Maximum # of Minimum # of Minimum Frames Having Noise PhotonsSignal Photons Required a Photon Count (FAR = 0.1) (PD = 0.99) SNR 4 0.23 15 5 0.4 4 10 6 0.8 4 6.7 7 1.0 5 6.3 8 1.0 5 5 9 1.2 5.7 4.8 10 1.46.5 4.6 11 1.75 7.3 4.2 12 2.05 8.2 4 13 2.42 9.2 3.8 14 2.85 10.5 3.715 3.3 12 3.6

Careful examination of Table 3 shows that, for a solar background ofapproximately 0.5 photons, a threshold value of about 4 signal photonsis required at receiver 104. Further, approximately 1 photon due tosolar background can be tolerated with 5 signal photons at a thresholdlevel of 8 detection frames where N=20. One skilled in the art willrecognize that the threshold value is consideration of system design anddesired performance and that the threshold value can be any valuesufficient to achieve a desired system performance.

At operation 803, processor 106 computes the distance between vehicle108 and objects located along detection axis 112.

At operation 804, system 700 rotates detection axis 112 to interrogate adifferent portion of detection field 114.

At operation 805, processor 106 develops a map of detection field 114.

It is to be understood that the disclosure teaches just one example ofthe illustrative embodiment and that many variations of the inventioncan easily be devised by those skilled in the art after reading thisdisclosure and that the scope of the present invention is to bedetermined by the following claims.

What is claimed is:
 1. A method for developing a map of objects relativeto a first location, the method comprising: transmitting a first opticalpulse from a transmitter at a first time, the first optical pulse havinga wavelength within the range of approximately 1350 nm to approximately1475 nm; enabling a receiver to detect a first reflection only when itoriginates from within a first zone of a plurality thereof, wherein theplurality of zones collectively defines a detection field, and whereinthe receiver is enabled to detect the first reflection by operationscomprising; (i) providing the receiver such that it includes asingle-photon detector; (ii) arming the single-photon detector at athird time, the third time being based on the time-of-flight for aphoton travelling from the transmitter to the receiver via a first pointin the first zone; and (iii) disarming the single-photon detector at afourth time, the fourth time being based on the time-of-flight for aphoton travelling from the transmitter to the receiver via a secondpoint in the first zone; detecting the first reflection of the firstoptical pulse from a first object within the detection field, whereinthe first reflection is detected at the receiver at a second time; andcomputing a distance between the first location and the first objectbased on the difference between the first time and the second time. 2.The method of claim 1, wherein the first optical pulse is transmittedalong a first direction and the first point is the nearest point to thefirst location along the first direction and the second point is thefurthest point to first location along the first direction.
 3. Themethod of claim 1 further comprising enabling the receiver to detect thereflection only when it originates from within a second zone of theplurality thereof.
 4. The method of claim 1 further comprisingco-locating the transmitter and the receiver at the first location. 5.The method of claim 1 further comprising transmitting a second opticalpulse from the transmitter at a third time, wherein the time between thefirst time and the third time is based the size of the detection field.6. The method of claim 1 wherein the first optical pulse is transmittedsuch that it has a wavelength within the range of approximately 1350 nmto approximately 1420 nm.
 7. The method of claim 1 wherein the firstoptical pulse is transmitted such that it has a wavelength within therange of approximately 1350 nm to approximately 1390 nm.
 8. The methodof claim 1 wherein the first optical pulse is transmitted such that ithas a wavelength within the range of approximately 1435 nm toapproximately 1475 nm.
 9. A system comprising: a transmitter operativefor providing an optical pulse from a first location at a first time,the optical pulse having a wavelength within the range of approximately1350 nm to approximately 1475 nm; a receiver including a single-photondetector operative for detecting a reflection of the optical pulse froman object within a detection field at a second time; and a processoroperative for: (i) logically separating the detection field into aplurality of detection zones that are arranged contiguously along anaxis; (ii) establishing a detection frame having a start time that isbased on the first time and a stop time based on a maximum distance ofthe detection field; (iii) establishing a plurality of sub-gate periodsthat collectively define the detection frame, wherein each sub-gateperiod corresponds to a different detection zone of the pluralitythereof; and (iv) for each sub-gate period, enabling the receiver todetect the reflection only when it originates from its correspondingdetection zone; and (v) estimating a separation distance between asecond location and the object based the difference between the firsttime and the second time, wherein the second location is based on thefirst location.
 10. The system of claim 9, wherein, for each sub-gateperiod, the processor enables the receiver to detect the reflection thatoriginates from its corresponding detection zone by operationscomprising: arming the single-photon detector at a third time that isbased on a first position within the detection zone, the first positionbeing a position along the axis that is closest to the first location;and disarming the single-photon detector at a fourth time that is basedon a second position within the detection zone, the second positionbeing a position along the axis that is furthest from the firstlocation.
 11. The system of claim 9 wherein each of the plurality ofsub-gate periods has a duration based on a dimension of a zone of theplurality thereof, and wherein the dimension is a depth along adetection axis.
 12. The system of claim 9 further comprising a scanneroperative for enabling the transmitter to transmit the optical pulsealong a detection axis, wherein the scanner is further operative forchanging the orientation of the detection axis in at least onedimension.
 13. The method of claim 9 wherein the optical pulse isprovided such that it has a wavelength within the range of approximately1350 nm to approximately 1420 nm.
 14. The method of claim 9 wherein theoptical pulse is provided such that it has a wavelength within the rangeof approximately 1350 nm to approximately 1390 nm.
 15. The method ofclaim 9 wherein the optical pulse is provided such that it has awavelength within the range of approximately 1435 nm to approximately1475 nm.
 16. A method for developing a map of objects within a detectionfield having a plurality of M detection zones that are contiguouslyarranged along a detection axis, the method including interrogating thedetection axis by operations comprising: (1) establishing a firstdetection frame; (2) transmitting an optical pulse from a transmitter ata first location at a first time, the optical pulse being characterizedby a wavelength within the range of approximately 1350 nm toapproximately 1475 nm; (3) separating the detection field into M zones,zone-1 through zone-M; and (4) for each detection zone j, where j=1through M, (a) enabling a receiver to detect a reflection-j of theoptical pulse only when the reflection originates within zone j; and (b)if a reflection-j is detected at the receiver; (i) establishing adetection time-j based on the time when reflection-j is received; and(ii) computing a separation distance between an object j, the separationdistance being based a difference between the first time and detectiontime j.
 17. The method of claim 16 further comprising, for each detectedreflection-j, establishing a position for object-j within the detectionfield.
 18. The method of claim 16 further comprising providing thetransmitter and receiver such that they are substantially co-located.19. The method of claim 16 wherein the optical pulse is provided suchthat it has a wavelength within the range of approximately 1350 nm toapproximately 1420 nm.
 20. The method of claim 16 wherein the opticalpulse is provided such that it has a wavelength within the range ofapproximately 1350 nm to approximately 1390 nm.
 21. The method of claim16 wherein the optical pulse is provided such that it has a wavelengthwithin the range of approximately 1435 nm to approximately 1475 nm.